See if this helps.
The topic that dmiscannon linked in is your best bet. I frequently get that in my log files for tests where I attempt to click on something before it is ready to be clicked on. One of our web applications doesn't actually "destroy" modal forms when they go away, it simply sets the height and width to zero. So, the form is always present and "Exists" is always true. It is, also, technically always visible. So, when we want to detect if the form is on screen, we check the "Width" property. If it is greater than zero, we then can act against it. We use the "WaitProperty" method in a loop. While Width is zero, we keep looping until either the width changes or we hit a maximum time out. You might want to consider implementing something like that.
